Electrolytes and acid-base

what are electrolytes

substance that ionises when dissolved in water

sodium Na+

potassium K+

chloride Cl-

where are they distributed

distributed within body water

around 60% body weight lean adult

• intracellular fluid (ICF) - around 40% body weight

• Extracellular fluid (ECF) ~20% body weight

• ▪ Interstitial fluid (15%)

• ▪ Plasma (5%)

60:40:20 rule

Changes in body water distribution may affect electrolyte values

Interpretation of electrolyte measurements should take into consideration ECF volume

where are they distributed

how they are maintained in health

kidneys

Distribution of electrolytes depends on

• Osmolality- distribution of solutes

• Electroneutrality — Acid base status-anion gap

osmolality

All body fluids seek an isotonic balance with each other in health

Electrolytes (primarily Na+ )

• Bicarbonate (HCO3 - )

• Glucose

• Urea

regulation of sodium through diff means….

Regulation of blood volume

Hypovolaemia (low blood volume-low blood pressure - so make fluid concentrated to increase osmosis)

▪ Activation of RAS-> Angiotensin II- > resorption of Na+, Cl - and H2O proximal tubule

▪ Aldosterone stimulates active absorption of Na+ in the collecting tubules

▪ ADH secreted-> resorption of Na+, Cl - and H2O medullary thick loop of Henle

Increased blood flow

▪ ANP (lowers blood pressure and decreases blood volume) -> promotes Na+ excretion in distal nephron

Regulation of plasma osmolality

Hyperosmolality (too much protein in fluid/blood)

▪ Thirst centres stimulated - > increased H2O intake

▪ ADH released -> increased resorption of H2O

Hypoosmolality

▪ Decreased H2O intake

▪ ADH secretion diminished -> less resorption of H2O

regulation of potassium - cellular uptake

regulation of potassium through renal excretion

Aldosterone stimulates the

• Secretion of K+ by principal cells of the collecting tubules

High flow rate (of aldosterone) promotes excretion

Low flow rate inhibits excretion

Hypochloraemia (very low Cl)

• Resorption of Na+ without Cl leads to secretion of K+ (because Cl- is negative and Na+ is positive, when they leave the urine together it keeps the electric balance neutral, when Cl- is low Na+ keeps the urine/ lumen negatively charged, pulling magnet K+ out of cell into urine)

just go over this with gemini icba ffs

alterations in potassium

Acidoses and Alkaloses affect serum K+ Inorganic or mineral acidosis may cause K+ to shift out of the cells -> hyperkalaemia in peripheral blood

Organic acidoses eg lactate or ketoacidosis typically will not

• When lactate or ketone body enters the cell H+ goes with them

• Acidaemia may promote movement of K+ out of cells, but K+ is excreted in urine with the anions

Metabolic alkalosis may cause mild hypokalaemia

how do we measure them

Ion Selective Electrode

• The ISE has a selective membrane which only allows the ion of interest to enter.

• The difference between the two electrodes is calculated and the concentration of the ion is determined from this

if lipid is present then electrolytes are not in the lipid fraction BUT the machine will calculate as if all the plasma is in the aqueous phase

Except when it is a direct ISE

when platelets clot what happens

K+ is released

Thrombocytosis may result in higher serum K+

Prolonged contact of erythrocytes and serum (>2hours) may also increase K+ particularly with some dog breeds, horses and ruminants

what happens in disease

Solute loss in excess of water-loss of hypertonic fluids

Isotonic loss

Water loss in excess of solute- loss of hypotonic fluids

hypernatraemia

H2O deficient

• Inadequate intake

• Pure H2O loss

• H2O loss> Na+loss

▪ Renal

▪ Alimentary

Na+ excess

• Salt poisoning

• Administration of hypertonic saline or sodium bicarbonate

• Decreased renal excretion-hyperaldosteronism

normonatraemia when dehydrated

Net loss of isotonic fluids

• Alimentary Loss

• Renal Loss

• Skin loss-sweating horses

Net retention of isotonic fluids leading to oedma or transudate

• Congestive heart failure

• Hepatic cirrhosis

• Nephrotic Syndrome

hyponatraemia

Na+ deficit Na+loss >H2O loss

• Alimentary

• Renal

• Cutaneous

• Third space

H2O excess

• Oedematous disorders

Shift of H2O from ICF to ECF

• Hyperglycaemia

Shift of Na+ from ECF to ICF

• Acute muscle damage

Shift of Na+ from intravascular to extravascular

• Uroperitoneum

hyperkalaemia

Shifting of K+ from ICF to ECF

• Metabolic acidosis (inorganic acids)

• Rhabdomyolysis or other muscle damage

• Massive tissue necrosis

Increased total K+

• Decreased renal excretion

▪ Renal insufficiency particularly oliguric or anuric CKD

▪ Urinary tract obstruction or leakage

▪ Addisons

▪ Trimethoprim

Increased intake

???

• Peritoneal effusions

hypokalaemia

Shifting of K+ from ECF to ICF

• Metabolic alkalosis with alkalaemia

• (Rapid) increase in insulin activity

• Endotoxaemia

Decreased total body K+

• Decreased intake

• Increased renal loss

• Increased alimentary loss

???

• Hypokalaemic renal disease in cats

chloride

Generally will follow sodium

Will also be affected by bicarbonate levels

If normochloraemia present with hypernatraemia may be increased anion gap

• Lactic acidosis

• Ketoacidosis

• Renal failure

• Ethylene glycol or methanol poisoning

overview

Multiple processes affect electrolyte measurements

The measured electrolytes in the serum are only a small part of the total body values

Values should be interpreted in conjunction with

• Knowledge of volaemic state

• Knowledge of acid base status

• In conjunction with clinical presentation and other biochemistry parameters

image example right bar on D - increase of HCO-3 and decrease in Cl- = acidosis?

measurement of acid-base

types of acid-base disturbance

Disorders may be a combination of primary, and compensatory Mixed acid base disorders are less common but may also be seen.

blood gas data in simple acid-base disorders

algorithmic approach